Imaging devices typically employ a sensor to detect light, and in response, generate an electrical signal representative of the light incident on the sensor. The sensor typically employs a light sensing element such as a photodiode, and associated circuitry for selectively reading out the electric signal provided by the light sensing element. For example, light photons impinge upon the photodiode and are converted into electrons that constitute the electrical signal. Currently there are several known types of solid-state imaging devices. The two primary types of imaging devices are CCD (charge coupled device) devices and CMOS devices.
A typical four transistor (4T) CMOS imager pixel cell 10 is shown in FIG. 1. The pixel cell 10 includes a photosensor 12 (e.g., photodiode, photogate, etc.), transfer transistor 14, floating diffusion region FD, reset transistor 16, source follower transistor 18 and row select transistor 20. The photosensor 12 is connected to the floating diffusion region FD by the transfer transistor 14 when the transfer transistor 14 is activated by a transfer gate control signal TX.
The reset transistor 16 is connected between the floating diffusion region FD and an array pixel supply voltage Vaa_pix. A reset control signal RST is used to activate the reset transistor 16, which resets the floating diffusion region FD to the array pixel supply voltage Vaa_pix level as is known in the art.
The source follower transistor 18 has its gate connected to the floating diffusion region FD and is connected between the array pixel supply voltage Vaa_pix and the row select transistor 20. The source follower transistor 18 converts the charge stored at the floating diffusion region FD into an electrical output voltage signal Vout. The row select transistor 20 is controllable by a row select signal SEL for selectively connecting the source follower transistor 18 and its output voltage signal Vout to a column line 22 of a pixel array.
One common problem associated with solid-state imaging devices, including CCD and CMOS imaging devices, is consistency in a pixel cell's responsivity to light incident on the light sensing element across an array of pixel cells. For example, the responsivity of a first pixel cell will be different from a second pixel cell's responsivity. Consequently, the signal to noise (S/N) ratio of the pixel cells will not be equal which affects the overall dynamic range of the pixel cell array.
Image sensor arrays have a characteristic light dynamic range. Light dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. It is desirable to have an image sensor with a high light dynamic range to image scenes that generate high light dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
The electrical dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of the noise under dark conditions. Dynamic range is limited on an upper end by the charge saturation level of the image sensor, and on a lower end, by noise imposed limitations and/or quantization limits of the analog to digital converter used to produce the digital image. When the light dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene, e.g., by having a low light saturation level, the full range of the image scene is not reproduced.
As pixel cell size is scaled down, so is the size of the photo-conversion device. Therefore, the amount of charge the photo-conversion device and pixel cell can accumulate is reduced, degrading the image sensor's dynamic range. There are several approaches to improve dynamic range, one of which utilizes dual integration periods. As indicated above, another approach would be to add transistors to the pixel cell. Since it may be difficult to implement additional transistors inside a pixel while at the same time maintaining a small pixel size, the dual integration period approach is more desirable because the pixel cell can remain the same and only pulse timing related modifications would be needed.
It is desirable for an imager device to be capable of imaging in the presence of a high level of dynamic range in the brightness of the light incident upon pixel cells. It is also desirable for the imaging to be performed as a linear function of incident light. However, for any given integration period, in the presence of incident light having a high level of brightness, there is a danger of overexposure due to the photosensor 12 producing too many photo-generated charges. Conversely, in the presence of incident light having a low level of brightness, there is a danger of underexposure due to the photosensor 12 producing too few photo-generated charges.
Pixel cells having photosensors with the greatest sensitivity will become saturated first. Once a pixel cell saturates, the exposure to light is stopped to avoid blooming and other saturation artifacts. Conventional pixel arrays determine light intensity by illuminating the detector in the active area and determining light intensity simultaneous with image acquisition. As mentioned above, blooming or saturation is a problem that occurs when too many photons strike a particular pixel cell and overflow into adjacent pixel cells, which causes the adjacent pixel cells to incorrectly sense the image. When light intensity is determined in the active area pixel cells, there is a greater probability of blooming or saturation occurring on individual pixel cells because the detection of light intensity is occurring simultaneously as the active area pixel cells are receiving light.
What is needed, therefore, is an image sensor which achieves an improved dynamic range, and can be implemented using conventional pixel cells. An optimal pixel cell has a high dynamic range and low fixed pattern noise.